The present invention relates to a phase change type optical recording medium, and its fabrication method.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting or overwriting. One typical rewritable optical recording medium is of the phase change type wherein the recording layer is irradiated with laser beam to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reading. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simpler as compared with magneto-optical recording media.
Most optical recording media of the phase change type use chalcogenides such as Ge--Te base materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
In recent years, it has also been proposed to use compounds known as chalcopyrites.
Chalcopyrite compounds are extensively investigated as compound semiconductor materials, and applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described in Physics, Vol. 8, No. 8 (1987), page 441, Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), page 228, and other literature.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with antimony or Sb or bismuth or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See JP-A's 3-240590, 3-99884, 3-82593, 3-73384 and 4-151286.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A's 4-267192, 4-232779 and 6-166268 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms upon the crystallization of a recording layer.
When information is recorded on a phase change type optical recording medium, the recording layer is irradiated with laser beam of power (recording power) high enough to bring the recording layer to a temperature higher than the melting point thereof. The recording layer is melted at spots with the recording power applied thereon, and then clickly cooled so that recorded marks of amorphous nature can be formed. When the recorded marks are erased, on the other hand, the recording layer is irradiated with laser beam having such a relatively low power (erasing power) as to bring the temperature of the recording layer to a temperature higher than that the crystallization temperature thereof but lower than the melting point thereof. The recorded marks with the erasing power applied thereon go back to the amorphous state because they are slowly cooled down after heated to the temperature higher than their crystallization temperature. Thus, if the recording power and erasing power are applied on the recording layer while the intensity of the single light beam is modulated, it is then possible to achieve overwriting.
As overwriting is repeated, however, the phase change type optical recording medium based on an AgInTe.sub.2 or AgSbTe.sub.2 system undergoes C/N drops due to a decrease in the rate of erasure and signal degradation such as an increased jitter. This offers another problem; the number of overwritable cycles is limited to about 1,000.
JP-A 9-30124 has proposed a phase change type recording layer which comprises Ag, In, Sb and Te as primary components, and has a nitrogen content of 1 to 3 at % and an oxygen content of up to 3 at %. Therein it is stated that the advantage of the invention is that the degradation of C/N due to repeated overwriting can be reduced by preventing incorporation of oxygen in the recording layer. For means for reducing the content of oxygen in the recording layer, the publication refers to backing a plastic substrate, allowing a plastic substrate to stand alone in vacuum before the formation of the recording layer, etching a plastic substrate surface by sputtering, reducing the partial pressure of oxygen during the formation of the recording layer to 2.times.10.sup.-2 Pa or lower, and reducing the amount of oxygen in a sputtering target material used for the formation of the recording layer to 500 ppm or lower. In the example therein, it is stated that the greatest number of overwriting cycles is 70,000. However, the publication is silent about the criterion of in what cycles overwriting becomes impossible.
According to the results of experimentation conducted by the inventors, however, it is found that when the recording layer is formed by the method disclosed in JP-A 9-30124, the number of overwritable cycles is barely about 1,000, and so is little improved over that achieved so far in the art. It is here to be noted that by the number of overwritable cycles is intended the number of overwriting cycles at which the jitter is within 13%, as will be described later.
Furthermore, the means used therein for reducing the content of oxygen in the recording layer incur increases in the number of steps involved, production time, and target material cost. Thus, such means are not preferable because difficulty is experienced in producing the recording layer at low costs.